1. Field of the Invention
The present invention relates to an optical pickup for use with a compact disk player, a video disk player, or an optical disk memory apparatus, and in particular, to an optical apparatus capable of preventing a crosstalk between adjacent tracks on a recording surface of a disk without using an apodized filter.
2. Description of the Prior Art
FIG. 5 is a schematic diagram showing an arrangement of elements in an optical apparatus of an optical pickup for a conventional video disk player.
A laser beam emitted from a semiconductor laser 1 passes through a polarized beam splitter 2 and then travels through a collimate lens 3 to be collimated to a parallel light, which is then collected by an objective 5 so as to form a fine spot on a recording surface Da of an optical disk D. A diffraction grid 7 is disposed in a light path of the laser beam so as to attain three spots associated with the laser beam on the recording surface Da of the optical disk D. A reflection beam reflected from the optical disk D passes through a 1/4 wavelength plate 4 and then is reflected by the polarized beam splitter 2 to a direction having a right angle with respect to the incident direction of the laser beam; thereafter, the resultant beam passes through a light receiving lens 9 and is then detected by a light receiving element 6 such as a photodiode. The light receiving element 6 reads an RF signal recorded in a form of pits on the recording surface Da of the optical disk D.
In FIG. 6, reference numeral 8 indicates an apodized filter, which is integrally structured with a diffraction grid 7. The apodized filter 8 includes a transparent plate 8a at a central portion thereof, the plate 8a being transparent with respect to the beam and a semi-transparent plates 8b respectively disposed on both sides of the transparent plate 8a.
FIG. 7A shows a distribution of the amount of light of a laser beam incident to the objective 5 in a case where the apodized filter 8 is not provided; whereas FIG. 8A is a graph depicting a distribution of the amount of light of a laser beam incident to the objective 5 in a case where the apodized filter 8 is provided. In addition, FIG. 7B shows a distribution of the amount of light of a beam spot formed on the recording surface Da of the optical disk in a case where the apodized filter B is not provided: whereas FIG. 8B is a schematic diagram illustrating a distribution of the amount of light of a beam spot formed on the recording surface Da of the optical disk in a case where the apodized filter 8 is provided When the apodized filter 8 is not disposed, due to the diffraction phenomenon of the beam passed through the diffraction grid 7, a spot K.sub.1 is formed around a main spot K.sub.0 of the beam due to the primary diffraction as shown in FIG. 7B. Consequently, when the main spot K.sub.0 is scanning a track T.sub.2, there possibly occurs a case where the spot K.sub.1 associated with the primary diffraction covers pits of the adjacent tracks T.sub.1 and T.sub.3 respectively disposed on the right and left. In an apparatus such as a video disk player, since information is recorded in a form of analog signals, when the spot K.sub.1 associated with the primary diffraction covers the adjacent tracks T.sub.1 and T.sub.3, a crosstalk possibly occurs and the signal-to-noise (S/N) ratio is deteriorated in reproduction of the RF signal; consequently, there arises a phenomenon such as an occurrence of a noise in the screen.
When using the apodized filter 8, as shown in FIG. 6, the quasi-transparent plates 8b (with transmittivity of 50%, for example) restricts the light amount in the both sides of the effective portion 10 of the laser beam; consequently, as shown in FIG. 8A, of the laser beam incident to the objective 5, the amount of light on the right and left sides thereof is reduced. As a result, as shown in FIG. 8B, the beam spot associated with the primary diffraction is prevented from being formed in the beam spot attained on the recording surface Da of the optical disk D.
However, in the conventional optical apparatus of FIG. 5, when an apodized filter 8 is disposed as means to lower the intensity level of the spot due to the primary diffraction, the number of parts constituting the optical section is increased by the elements of the apodized filter 8 and hence the cost is increased.
Moreover, when the apodized filter 8 is employed, the numerical aperture of the objective 5 is substantially varied because of the quasi-transparent plates 8b; as a result, the diameter of the spot .phi. of FIG. 8B is increased and the intensity level of the spot is lowered. The decrease in the intensity level of the spot causes lowering of the detection level of the light receiving element 6, and the increase in the spot diameter .phi. lowers the degree of modulation.
Furthermore, the utilization of the apodized lens leads to an increase of the aberration associated with the quasi-transparent plates 8b.